There is a type of inkjet head which takes a partition wall of two adjacent pressure chambers as an actuator. In such a type of inkjet head, when a driving pulse signal including an expansion pulse and a contraction pulse is applied to the actuator, the partition wall deforms in a direction in which the pressure chamber is expanded or in a direction in which the pressure chamber is contracted. Then, pressure vibration is generated in the pressure chamber due to the volume change, and ink drops are ejected from a nozzle communicating with the pressure chamber.
In this way, since the inkjet head enables the ink drops to be ejected from the nozzle by deforming the partition wall of the pressure chambers, it is not possible to simultaneously eject the ink drops from adjacent nozzles which communicate with the adjacent pressure chambers respectively. Thus, the inkjet head divides the pressure chambers into, for example, three groups every third pressure chamber, and changes the phase of the driving pulse signal for each group. According to image patterns, three states are generated, which includes: one state where one nozzle ejects ink and the other nozzles do not eject ink (hereinafter, referred to as a single-nozzle driving state), one state where nozzles ejecting ink belong to any group and ink is not ejected from nozzles belonging to other groups (hereinafter, referred to as a multi-nozzle simultaneous driving state), and one state where ink is ejected from nozzles belonging to at least two groups at time division (hereinafter, referred to as a continuous multi-nozzle driving state).
The inkjet head adopts a multi-drop method adjusting the number of the ink drops ejected from one nozzle in case of carrying out gradation printing. In a case of adopting the multi-drop method, the ejection speed of the ink drops after the second drop is fastened due to the residual pressure vibration of the ink drop just ejected. However, since the pressure vibration is applied in a state where a meniscus is still, the ejection speed of the first drop of the ink drops is slower than that of the ink drops following the second drop. There is a technology to increase the ejection speed of the first drop by applying an auxiliary pulse signal (boost pulse) for amplifying the pressure vibration of the pressure chamber before the driving pulse signal for enabling the first drop to be ejected.
In a case of the multi-nozzle simultaneous driving state, there is a disadvantage that the ejection speed of the second drop is slower than that of the first drop, and thus the second drop is separated from the first drop and impacts on the first drop.